Increased cell proliferation and loss of differentiation have long been recognized as two major factors that promote neoplastic transformation and progression. This understanding has contributed tremendously to the development of antineoplastic treatments aimed at suppressing cell proliferation. Antineoplastic therapies, including radiation and chemotherapeutic agents, ultimately eliminate tumor cells by the induction of apoptosis, a physiological process of cell destruction. Accumulating evidence suggests that expression of oncogenes sensitizes many primary tumors to apoptotic cell death as compared to their normal counterparts, which provides a critical therapeutic window for treatment. However, this therapeutic opportunity is eroded by antiapoptotic mechanisms that are conferred by mutations accumulated during the transformation process or induced upon treatment.
In metazoans oncogenic alterations often sensitize cells to apoptotic stimuli. For example, expression of oncogenic c-myc or the adenovirus early region 1A can increase cellular susceptibility to apoptosis in circumstances such as growth factor deprivation. This oncogenic-induced sensitization serves as a physiological barrier against tumor development by limiting the expansion of affected cells. Such sensitization to apoptosis by oncogenes also provides a therapeutic window for treating many tumors with anticancer agents. However, it is widely believed that host tumor surveillance mechanisms select for upregulation of antiapoptetic mechanisms during the process of transformation decreasing the therapeutic benefit of conventional anti-cancer drugs. Accordingly, by inhibiting critical antiapoptotic mechanisms, sensitivity of tumor cells to therapy-induced apoptosis may be restored.
14-3-3 proteins are dimeric, phosphoserine-binding molecules that interact with a number of phospho-proteins involved in controlling cell death and proliferation. 14-3-3 polypeptides supports cell survival by inhibiting the death promoting activity of its associated proapoptotic partners. One prominent target of the 14-3-3 polypeptide is Bad. Bad is a proapoptotic member of the Bcl-2 family of apoptosis regulators. Interestingly, Bad is phosphorylated by activated Akt and other kinases which generates a 14-3-3 recognition site, leading to Bad/14-3-3 complex formation. Thus, the phosphorylation of Bad couples multiple survival signaling pathways to the cell death machinery. Bad is not the only 14-3-3 target with death-promoting activity. ASK1, a Ser/Thr kinase, is a critical element of a death signaling pathway initiated by TNFα, Fas activation, and the chemotherapeutic agents paclitaxel and cisplatin. 14-3-3 binding suppresses the death-promoting activity of ASK1.
Inhibition of Bad-, ASK-, and FKHRL1-induced apoptosis by 14-3-3 raises the possibility that 14-3-3 functions as an antiapoptotic factor (Brunet et al. (1999) Cell 96:857-868; Zha et al. (1996) Cell 87:619-628; and Zhang et al. (1999) Proc. Natl. Acad. Sci. 96:8511-8515, all of which are herein incorporated by reference). Because 14-3-3 interacts with a large array of ligands involved in both cell death and cell survival, 14-3-3 may be part of a general antiapoptotic mechanism essential for cell survival. 14-3-3 could support cell survival both by suppressing the activity of proapoptotic proteins and by promoting the activity of antiapoptotic proteins.
Given the shortcomings of current chemotherapy and irradiation, namely the lack of response and resistance or tolerance to the various antineoplastic agents, there is a need for developing additional forms of treatment that can enhance a response of a neoplastic cell to the antineoplastic therapies. The present invention provides a novel method of treating a neoplastic disorder by modulating the activity of a 14-3-3 polypeptide.